Carbo-Benzene's Aromaticity, Before and Beyond: A Tribute to Nozoe

[6]Cyclo-para-phenylmethine: An Analog of Benzene Showing Global Aromaticity and Open-Shell Diradical Character

We report a [6]cyclo-para-phenylmethine ([6]CPPM) macrocycle that shows benzene-like electronic properties. Its mesityl derivative, [6]CPPM-Mes, was isolated in crystalline form. X-ray analysis reveals a C_2h symmetry, and the bond lengths of the benzenoid/quinoid rings are averaged via resonance. One averaged ¹H NMR peak for the protons on the backbone was observed at room temperature, but it was split into one shielded and one deshielded resonance below 198 K, consistent with its globally aromatic character with a dominant 30π conjugation pathway along the periphery. It exhibits open-shell diradical character with a moderate singlet-triplet energy gap (ΔE_S-T = -6.23 kcal/mol). Its dication is antiaromatic and open-shell, showing a smaller ΔE_S-T value (-4.18 kcal/mol). Overall, [6]CPPM behaves like an open-shell aromatic "super-benzene".

Steric/π-Electronic Insulation of the carbo-Benzene Ring: Dramatic Effects of tert-Butyl versus Phenyl Crowns on Geometric, Chromophoric, Redox, and Magnetic Properties

Hexa-tert-butyl-carbo-benzene (C₁₈ tBu₆ ) and three phenylated counterparts (C₁₈ tBu_m Ph_6-m ; m=4, 2) have been synthesized. The peralkylated version (m=6) provides experimental access to intrinsic features of the insulated C₁₈ core independently from the influence of π-conjugated substituent. Over the series, structural, spectroscopic, and electrochemical properties are compared with those of the hexaphenylated reference (m=0). Anchoring tBu substituents at the C₁₈ macrocycle is shown to enhance stability and solubility, and to dramatically modify UV/Vis absorption and redox properties. Whereas all carbo-benzenes reported previously were obtained as dark-reddish/greenish solids, crystals and solutions of C₁₈ tBu₆ happen to be yellow (λ_max =379 vs. 472 nm for C₁₈ Ph₆ ). In comparison to C₁₈ Ph₆ , the reduction of C₁₈ tBu₆ remains reversible, but occurs at twice as high an absolute potential (E_1/2 =-1.36 vs. -0.72 V). Systematic XRD analyses and DFT calculations show that the C₁₈ ring symmetry is the nearest to D_6h for m=6, which indicates a maximum geometric aromaticity. According to calculated nucleus-independent chemical shifts (NICS), the macrocyclic magnetic aromaticity is also maximum for C₁₈ tBu₆ : NICS(0)=-17.2 ppm versus (-18.0±0.1) ppm for the theoretical references C₁₈ H₆ and C₁₈ F₆ , and -13.5 ppm for C₁₈ Ph₆ . Accurate correlations of NICS(0) with experimentally recorded or calculated maximum UV/Vis absorption wavelengths, λ_max , and chemical hardness, η=E_LUMO -E_HOMO , are evidenced.

Reductive Aromatization/Dearomatization and Elimination Reactions to Access Conjugated Polycyclic Hydrocarbons, Heteroacenes, and Cumulenes

Acenes, heteroacenes, conjugated polycyclic hydrocarbons, and polycyclic aromatic hydrocarbons (collectively referred to in this review as conjugated polycyclic molecules, CPMs) have fascinated chemists since they were first isolated and synthesized in the mid 19th century. Most recently, these compounds have shown significant promise as the active components in organic devices (e.g., solar cells, thin-film transistors, light-emitting diodes, etc.), and, since 2001, a plethora of publications detail synthetic strategies to produce CPMs. In this review, we discuss reductive aromatization, reductive dearomatization, and elimination/extrusion reactions used to form CPMs. After a brief discussion on early methods to synthesize CPMs, we detail the use of reagents used for the reductive (de)aromatization of precursors containing 1,4-diols/diethers, including SnCl₂ and iodide (I^- ). Extension of these methods to carbomers and cumulenes is briefly discussed. We then describe low-valent metal species used to reduce endoxides to CPMs, and discuss the methods to directly reduce acenediones and acenones to the respective acene. In the final section, we describe methods used to affect aromatization to the desired CPM via extrusion of small, volatile molecules.

Selective access to p-dialkyl-carbo-benzenes from a [6]pericyclynedione: the n-butyl nucleophile model for a metal switch study

The synthesis, spectroscopic properties, comparative electrochemical behavior in CHCl3 vs CH2Cl2, and X-ray crystal structure of p-di-n-butyl-tetraphenyl-carbo-benzene are described. The selectivity of preparation of the ultimate [6]pericyclynediol precursor has been examined by comparing the reactivity of the [6]pericyclynedione substrate with four n-Bu-MXn nucleophiles involving more or less halogenated metal centers MXn (0 ≤ n ≤ 2): Li, MgBr, MgCl, CeCl2/LiCl. The cerium reagent is found to be the most efficient, giving approximately twice the yield given by Grignard reagents in the target diadduct (90% vs 51%–53%). The dibutyl-carbo-benzene product happens to be soluble in both CHCl3 and CH2Cl2: cyclic voltammograms of either solution exhibit almost identical peak potentials with reduced reversibility of the redox processes in CHCl3.

Lipidic Carbo-benzenes: Molecular Probes of Magnetic Anisotropy and Stacking Properties of α-Graphyne

Solubilization of the C₁₈ fundamental circuit of α-graphyne has been envisaged by decoration with aliphatic chains R = n-C_nH_2n+1. The synthesis and characterization of p-dialkyl-tetraphenyl-carbo-benzenes (n = 2, 8, 14, 20) are thus presented and compared to the monoalkyl series produced concomitantly. In both series, a dramatic enhancement of solubility in organic solvents (CH₂Cl₂, CHCl₃) is observed for n ≥ 8, and in the dialkyl series, the melting-decomposition temperature of the solid products is shown to decrease linearly from 208 °C for n = 2 to 149 °C for n = 20. Fluoroalkyl analogues with R = n-C₈H₄F₁₃ are also described. The products display classical UV-vis electronic spectra of carbo-benzenes in solution (λ_max = 445.5 ± 1 nm, ε ≈ 200 000 L·mol^-1·cm^-1). They are also characterized by UV-vis absorption in the solid state, which is found to be correlated with the color and crystal packing. The methylene groups of R provide an experimental probe of the magnetic anisotropy and aromaticity of the C₁₈ ring through the progressive NMR shielding of the ¹H nuclei from ca. 4.70 to 1.25 ppm going away from the border of the ring (as far as 8 Å away). All alkyl-carbo-benzenes were also found to be highly crystalline. Seven of them have been characterized by X-ray diffraction analysis and the C₁₈ columnar packing compared in a systematic manner. Crystals of the diethyl and bistetradecyl derivatives, containing no solvent molecule, provided the first examples of direct π-stacking of carbo-benzene rings, with inter-ring distances very close to calculated interlayer distances in AB and ABC α-graphityne (3.255 and 3.206 Å vs 3.266 and 3.201 Å, respectively).

Hexaaryl-carbo-benzenes revisited: a novel synthetic route, crystallographic data, and prospects of electrochemical behavior

Hexaphenyl-carbo-benzene and a substituted homologue were prepared via an efficient route, and characterized by crystallography and voltammetry with the view of studying charge transport properties within thin films.

“Carbo-aromaticity” and novel carbo-aromatic compounds

Recent advances in experimental and theoretical studies ofcarbo-benzene derivatives, along with the proposition of a generalization of the definition of aromaticity to the two-membered π-rings of triple bonds, suggest relevance for the notion of “carbo-aromaticity”.